The present invention relates to an optical power stabilizing device for always generating a desired stabilized light power from a light source without subjecting to the effect of change in ambient temperature or deterioration of the light source, which generate a light output power in accordance with input signals such as a pulse signal, an analog signal or the like.
As an optical sensor, a control technology and an information processing technology utilizing light are developed, it is important to use a stabilized light power obtained by a light source. Generally, the optical power must be utilized in such a manner that not only a constant light power is continuously generated (hereinafter referred to as a continuous light), but also a pulsatory light or a modulated light (hereinafter referred to as a modulated light) is generated in accordance with its aim. In this case, these kinds of light are utilized by combinning them with time or by superimposing them on each other. While there are many kinds of light sources such as a semiconductor laser element, a discharge tube or the like which have respective inherent characteristic of light power and dependency of ambient temperature change or deterioration of element.
Particularly, the light source such as a semiconductor device, a discharge tube or the like has a nonlinear light power characteristics with respect to the drive current (voltage) for the light source. The non-linear characteristics of the light output power is to be understood to mean a curved line characteristics which show zero or very weak light output power at very small drive current (voltage) and an abruptly increased light output power as the drive current (voltage) is increased. In order to obtain stabilized light output power from the light source having such a non-linear characteristics of the light output power, it is necessary to perform the stabilization of the light output power by controlling the drive current (voltage) suitably before and after a knee point of the curved line. As a typical light source having the nonlinear characteristic of the light output power, a semiconductor laser element is described by way of example with reference to the drawings, corresponding elements having been given the same reference numerals.
As shown in FIG. 1, the semiconductor laser element exhibits a non-linear characteristic of the light output power having a curved line portion with respect to a drive current I. The knee point is, generally, defined by a threshold current I.sub.th and the slope of a steep rising portion after the knee point is defined by a differential quantum efficiency .eta.. The light power characteristics of such a semiconductor laser element exhibits an increase of the threshold current I.sub.th and a decrease of the differential quantum efficiency .eta. due to an increase in ambient temperature of the laser element or a deterioration of the element. Such a variation of the light power characteristic is shown in FIG. 1 by C.sub.1 and C.sub.2. The effect of the temperature dependency or the deterioration dependency on such a light power characteristic is remarkable when the semiconductor laser element is driven by a pulse current to obtain a pulsatory light power. FIG. 1 also shows such an effect by light powers P.sub.1 and P.sub.2. This is the case that the semiconductor laser element is used as a light source of an optical memory device, the readout of the stored information is performed by a weak continuous oscillation mode of the laser element caused by a DC current I.sub.R, and the writing or erasing of information is performed by a strong pulse oscillation mode caused by a pulse current I.sub.W. As is seen from FIG. 1, when the light power characteristic is changed from C.sub.1 to C.sub.2 due to the deterioration and temperature rise of the laser element, the continuous oscillation mode (output component corresponding to the DC current I.sub.R) for readout can not often be obtained.
An optical power stabilizing device for generating a stabilized light power from a light source having such a non-linear characteristic has been proposed and disclosed in Japan Patent Laid-open No. 59-78588. Even in this case, however, the light power from the light source can only be stabilized by the detected output of an optical detector. Particularly, a biasing light quantity is changed by a peak value of the detected output, so that there are some problems unsolved, such as the necessity of controlling light power of the continuous light at a desired level with stability, the difficulty in following to duty change of the pulse signal, or the like.